However, some organization may not be very happy to give their users such large email limitations due to compliance requirements. For example, your organization may have a policy to restrict the send receive size based on employee’s seniority. Or you may control it depending on employee’s department.

Whatever it may be, as an administrator, you have the capability to lock down these large limitations by using transport rules. It can be either,

a limit on the maximum size of a single attachment

a limit on the total message size

a limit on both the size of a single attachment and the total mail size

This way you get more control over the behavior of the email. In this post I’m going to talk about the o, which you can utilize users’ group membership to control the total size of a single attachment and the total size of the email.

Exchange Online can only see mail enabled security groups or distribution groups. Therefore, you have to make sure the groups you are going to utilize for this are either mail enabled security groups or distribution groups. You can use any existing distribution group or if you create groups that are purely used for the mail control, and you do not want other users to be able to send emails, you can enable email moderation for these and hide them from your GAL.

Once you have groups created according to your requirement, head to mail flow -> rules.

Click + -> Create a new rule… Although it has Filter messages by size option, I’d always start fresh.

In this window, click on more options… link located to the bottom of the window. This will enable you to add multiple conditions and actions.

Perform below steps under *Apply this rule if…

Select The sender… -> is a member of this group

Pick the distribution group which you created earlier.

Click add condition to add another condition.

Select Any attachment… -> size is greater than or equal to

Specify the size of a single attachment in KBs.

Click add condition to add another condition.

Select The message… -> size is greater than or equal to

Specify the size of the total message in KBs.

Perform below steps under *Do the following…

Select Block the message… -> reject the message and include an explanation

Enter an explanation you would want users to see when their email gets blocked.

Although above steps are for sending, you can configure the same for receiving as well by modifying the step 4.1 as The recipient… -> is a member of this group.